Few practical methods for the preparation of unsymmetric dialkyl carbonates are known. A potentially useful approach is to react a symmetric dialkyl carbonate with an alcohol in the presence of a transesterification catalyst. Known transesterification catalysts include Lewis acids, soluble tin compounds, cyclic amines, N,N-dimethylaminopyridine, organotitanates, and acidic ion-exchange resins.
Unfortunately, the synthesis of unsymmetric dialkyl carbonates, particularly those derived from the reaction of a symmetric dialkyl carbonate and a tertiary alcohol, is often hampered by poor reactivity, difficult catalyst recovery, and/or unwanted side reactions. For example, I have found that the preparation of methyl tert-butyl carbonate from dimethyl carbonate and tert-butyl alcohol in the presence of "Amberlyst 15" resin (a product of Rohm and Haas Company) is unsatisfactory because tert-butyl alcohol rapidly dehydrates to isobutylene under these conditions.
Inorganic carbonates, alcoholates, and hydroxides also catalyze transesterifications of dialkyl carbonates (see, for example, European Patent Application No. 0274953 and Ind. Enq. Chem. Res. 27 (1988) 1565; 28 (1989) 881). Crown ethers or polyethylene glycols can be used in combination with the inorganic bases to enhance reactivity. Lithium, sodium, and potassium compounds are taught; cesium compounds are not taught. The target products are typically symmetric dialkyl carbonates.
U.S. Pat. No. 3,642,858 lists basic cesium compounds as suitable catalysts for the preparation of dialkyl carbonates from the reaction of cyclic carbonates (e.g., ethylene carbonate) with non-tertiary hydroxy-containing compounds. The products exemplified in the reference are all symmetric carbonates, and only sodium catalysts are shown.
I have found that basic lithium, potassium, and sodium compounds are generally unsatisfactory transesterification catalysts for the preparation of unsymmetric dialkyl carbonates from symmetric dialkyl carbonates, and that this is particularly true for the preparation of unsymmetric dialkyl carbonates derived in part from tertiary alcohols. In view of the value of certain unsymmetric dialkyl carbonates as octane enhancers for fuels, an improved synthesis of these compounds is needed.